Method of fabricating a stator assembly for a non-static cogging brushless DC motor

ABSTRACT

A stator assembly for a non-static cogging brushless DC motor includes a hollow coil forming member fabricated on a nonconductive and nonmagnetic material having an array of generally radially extending teeth. Each tooth has a pair of generally radially extending sidewalls that extend substantially the entire axial length of the coil forming member. A plurality of coils is wound on the teeth of the coil forming member so that the coils are disposed in surrounding relationship with respect to the radially extending sidewalls of the teeth. The teeth on the coil forming member may extend either radially inwardly or radially outwardly thereof.

This is a continuation of application Ser. No. 07/430,140 filed Nov. 1,1989, now abandoned, which is a division of application Ser. No.07/317,213, filed Feb. 28, 1989, now U.S. Pat. No. 5,013,953.

FIELD OF THE INVENTION

The present invention relates, in general, to a brushless DC motor andin particular to a brushless DC motor having a stator assembly with anonconductive and nonmagnetic coil forming member and to a method forfabricating a stator assembly for a brushless DC motor having the same.

DESCRIPTION OF THE PRIOR ART

With reference to FIGS. 1 and 2 shown are, respectively, an isometricview of a stator member M and an end elevational view (partly insection) of a stator assembly S including the stator member M, for abrushless DC motor. The stator member M is a generally cylindricalmember having a plurality of generally radially inwardly extending teethT. The teeth T have generally radially inwardly extending sidewalls Wthat extend axially along the length of the stator member M. The statormember M is typically formed from joined plural ferromagnetic lamellaeL. Stator coils C (FIG. 2) are wound around the teeth T in abuttingrelationship to the sidewalls W of the teeth T to form the statorassembly S. The teeth T accurately position the coils C on the interiorof the stator member M.

To form a brushless DC motor, a rotor R shown in dot-dash lines in FIG.2 having permanent magnets P thereon is inserted into the statorassembly S. Application of a DC voltage to the coils C in accordancewith a predetermined pattern as governed by a suitable switching matrixcauses the rotor R to rotate within the stator assembly S.

In a brushless DC motor having a stator assembly S as shown in FIG. 2, aphenomenon known as "static cogging" can occur. This phenomenon shouldnot be confused with the term "cogging" as applied to the normal torqueripple inherent in a brushless DC motor. "Static cogging" is the abruptshifting of the rotor R during very low speed operation caused by thepreferential attraction of the magnets P on the rotor R to the mostproximal one of the ferromagnetic teeth T of the stator assembly S. U.S.Pat. No. 4,687,977 (Brahmavar) discloses a brushless DC motor thatappears susceptible to static cogging. If such a motor is used in acentrifuge instrument intended for the separation of gradients, staticcogging can be deleterious in that it can result in remixing ofseparated fractions of the gradient.

Static cogging can be avoided if the coils of the motor are located on agenerally smooth interior walled stator member. U.S. Pat. No. 4,563,808(Lender) discloses two alternate methods for winding the stator coils onsuch a smooth interior walled stator member. Shown in FIGS. 3 and 4 arerespectively an isometric and an end elevational view of a prior artsupport member G for a stator assembly S for a non-static coggingbrushless DC motor. The stator support member G is a generally elongatedcylindrical member formed of a plurality ferromagnetic lamellae L. Thestator support member G is generally hollow and has an exterior surfaceE and an interior surface I. The interior surface I defines a borehaving a predetermined interior dimension D_(I). The exterior surface Eof the member G defines a predetermined exterior dimension D_(E). Boththe exterior surface E and the interior surface I of the support memberG are smooth.

Both methods disclosed in the above-referenced Lender patent utilizetemporary supports on which the coils are wound. The advantage of such atechnique is that the winding of the coils is accomplished usingconventional winding machines. However, this method is believed to bedisadvantageous because the expansion of a split cylindrical member usedto compress the stator coil radially against the inside wall of thestator member is believed to cause the position of the coils to shift.

U.S. Pat. No. 4,556,811 (Hendricks) discloses a coil form for a statorassembly for a brushless DC motor that accurately positions the statorcoils. The stator coils are wound so as to surround the interior andexterior walls of the cylindrical coil form. However, such a coil formis believed disadvantageous because coils cannot be wound on it usingconventional winding machines.

U.S. Pat. No. 3,932,929 (Hallerback et al.) and U.S. Pat. No. 4,321,494(MacNab) both disclose stator members for brushless DC motors. The formof the stator in each of these patents appears to preclude the windingof the coils using conventional winding machines.

Accordingly, in view of the foregoing it is believed advantageous toprovide a stator assembly for a non-static cogging brushless DC motorand a method of fabricating the same that accurately and reproduciblypositions the stator coils without compromising the use of aconventional winding apparatus.

SUMMARY OF THE INVENTION

The present invention relates to a stator assembly for a non-staticcogging brushless DC motor and a method of manufacturing the same. Thestator assembly includes a hollow coil forming member mounted on asupport member. The coil forming member is fabricated of a nonconductiveand nonmagnetic material and has an array of generally radiallyextending teeth. Each tooth has a pair of generally radially extendingsidewalls that extend substantially the entire axial length of the coilforming member. A plurality of coils is wound on the teeth of the coilforming member so that the coils are disposed in surroundingrelationship with respect to the radially extending sidewalls of theteeth. The teeth on the coil forming member may extend either radiallyinwardly or radially outwardly thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription thereof, taken in connection with the accompanying drawingswhich form a part of this application and in which:

FIG. 1 is an isometric view of a stator member for a brushless DC motorof the prior art;

FIG. 2 is an end elevational view of a stator assembly using the statormember of FIG. 1, the view of FIG. 2 being taken along view lines 2--2in FIG. 1 and having the segment indicated by dotted lines in FIG. 1removed;

FIGS. 3 and 4 are respectively an isometric and an end elevational viewof a prior art support member for a stator for a brushless DC motor;

FIGS. 5 and 6 are respectively an isometric and an end elevational viewof a first embodiment of a coil forming member in accordance with thepresent invention;

FIG. 7 is a sectional view of a stator assembly for a non-static coggingbrushless DC motor in accordance with the present invention having acoil forming member shown in FIGS. 5 and 6;

FIGS. 8 and 9 are respectively an isometric and an end elevational viewof an alternate embodiment of the coil forming member in accordance withthe present invention; and

FIG. 10 is a sectional view of a stator assembly for a non-staticcogging brushless DC motor in accordance with the present inventionhaving a coil forming member shown in FIGS. 8 and 9.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the following detailed description similar reference numeralsrefer to similar elements in all figures of the drawings.

FIGS. 5 and 6 are respectively an isometric and an end elevational viewof a first embodiment of a coil forming member 10 used to form a statorassembly 12 as shown in FIG. 7. In the stator assembly 12 shown in FIG.7 the coil forming member 10 is mounted to the interior surface I of thesupport member G (FIGS. 3 and 4). The support member G defines thestructural member on which is mounted the coil forming member 10 to formthe stator assembly 12 in accordance with the present invention.

The coil forming member 10 is a hollow, generally cylindrical memberhaving an interior surface 14 and an exterior surface 16. The exteriorsurface 16 is smooth and has a predetermined diametrical dimension 18that is substantially equal to the interior dimension D_(I) of thesupport member G. The coil forming member 10 is fabricated of anonconductive and nonmagnetic material. Preferably the material used toform the coil forming member 10 is a polymeric composition such as aurethane epoxy compound. The member 10 may be fabricated in anyconvenient fashion, as by injection molding or machining.

In the embodiment of the coil forming member 10 shown in FIGS. 6 and 7an array of generally radially extending teeth 20 is provided on theinterior surface 14. Each tooth 20 has a pair of generally radiallyextending sidewalls 22A, 22B. The sidewalls 22A, 22B extendsubstantially the entire axial length 24 of the coil forming member 10.Each of the teeth 20 terminates in a generally circumferentiallyextending tip surface 26.

A coil 30, comprising a plurality of turns 32 of an electricalconductor, is wound on the teeth 20 of the coil forming member 10. Eachof the plurality of turns 32, and thus the coil 30 so formed, isdisposed in surrounding relationship with respect to the radiallyextending sidewalls 22A, 22B of a tooth 20. The coils 30 are wound usinga standard coil winding apparatus. U.S. Pat. No. 4,563,808 (Lender)identifies a coil winding apparatus.

In FIG. 7 the stator assembly 12 includes the coil forming member 10mounted on the interior surface I of the support member G. It is thereheld by a layer 40 of an adhesive such as a two part urethane adhesivecompound. It should also be understood that in accordance with thepresent invention the coils 30 may be wound prior to insertion of thecoil forming member 10 into the support G. Alternatively the coilforming member 10 may be introduced into the support G and the resultingelement thereafter inserted into a winding apparatus and the coils 30then wound thereon. It should also be appreciated that the coil formingmember 10 may be alternatively mounted to the support member G by insertinjection molding the coil forming member to the support member usingthe support member as the insert. In this event the coils are woundafter the forming member is mounted to the support member.

FIGS. 8 and 9 respectively illustrate an isometric and an endelevational view of an alternate embodiment of the coil forming member10' in accordance with the present invention. FIG. 10 shows a statorassembly 12' in which the coil forming member 10' is mounted to theexterior surface E of the support member G.

The coil forming member 10' differs from the member 10 (FIGS. 5 and 6)in that the interior surface 14 is smooth while the teeth 20 extendradially outwardly from the outer surface 16 thereof. The interiordimension 19 of the coil forming member 10' is generally equal to theexterior dimension D_(E) of the support member G so that the coilforming member 10' may be mounted (in either manner discussed above) tothe exterior of the member G to form the stator assembly 12' shown inFIG. 10. As discussed above the coils 30 may be wound either prior to orsubsequent to the mounting of the member 10' on the support G.

Using the coil forming member 10 or 10' in accordance with the presentinvention results in a stator assembly 12 or 12', respectively, in whichthe coils 30 are able to be reproducibly and accurately equiangularlyspaced with respect to each other. Thus, in operation, a non-staticcogging brushless DC motor having a stator assembly 12 or 12' inaccordance with the present invention will be more efficient and exhibitless variation in torque response than a prior art non-static coggingbrushless DC motor.

Those skilled in the art, having the benefit of the teachings of thepresent invention may impart numerous modifications thereto. Thesemodifications are to be construed as lying within the scope of thepresent invention as defined by the appended claims.

What is claimed:
 1. A method of fabricating a stator assembly for abrushless DC motor comprising the steps of:first winding an electricalconductor on a hollow coil forming member having an array of generallyradially extending teeth, each tooth having a pair of generally radiallyextending sidewalls that extend substantially the entire axial length ofthe coil forming member such that the conductor takes the form of a coilhaving a plurality of turns with the turns being disposed in surroundingrelationship with respect to the radially extending sidewalls of theteeth, the hollow coil forming member having an interior surface and anexterior surface thereon, one of the interior or exterior surfaces beingsmooth, the coil forming member, including the teeth thereof, beingfabricated of a nonconductive and nonmagnetic material; and thereaftermounting the coil forming having the coil thereon on a support memberwith the smooth surface of the coil forming being positioned on thesupport member.
 2. The method of claim 1 wherein the support member hasan exterior surface thereon and wherein the interior surface of the coilforming member is the smooth surface, the coil forming member beingmounted with the smooth interior surface thereof on the exterior surfaceof the support member.
 3. The method of claim 1 wherein the supportmember is generally hollow and has an interior surface thereon andwherein the exterior surface of the coil forming member is the smoothsurface, the coil forming member being mounted with the smooth interiorsurface thereof on the interior surface of the support member.
 4. Amethod of fabricating a stator assembly for a brushless DC motorcomprising the steps of:first mounting to a support member a coilforming member having an array of generally radially extending teeth,each tooth having a pair of generally radially extending sidewalls thatextend substantially the entire axial length of the coil forming member,the hollow coil forming member having an interior surface and anexterior surface thereon, one of the interior or exterior surfaces beingsmooth, the coil forming member, including the teeth thereof, beingfabricated of a nonconductive and nonmagnetic material, the smoothsurface of the coil forming being positioned on the support member; andthereafter winding an electrical conductor on the coil forming membersuch that the conductor takes the form of a coil having a plurality ofturns with the turns being disposed in surrounding relationship withrespect to the radially extending sidewalls of the teeth.
 5. The methodof claim 4 wherein the support member has an exterior surface thereonand wherein the interior surface of the coil forming member is thesmooth surface, the coil forming member being mounted with the smoothinterior surface thereof on the exterior surface of the support member.6. The method of claim 4 wherein the support member is generally hollowand has an interior surface thereon and wherein the exterior surface ofthe coil forming member is the smooth surface, the coil forming memberbeing mounted with the smooth interior surface thereof on the interiorsurface of the support member.